There are quite a few innacuracies, although some of these are less innaccuracies and more new discoveries after the series was made.
Dimetrodon's habitat is pretty inaccurate, as well as possibly dimetrodon itself, however this is debatable as a lot of it is speculation.
As for the habitat, it was a lot more temperate and arboreal than arid desert.>>2606699
As mentioned here they include a giant spider in the Carboniferous episode, however it turned out it didn't look anything like they had animated. By the time the new research came out the episode had already been animated, so they just changed the name from megararchne to something else and carried on as normal. So take the whole giant spider episode with a pinch of salt.